The present invention relates to an improved lubricating fluid and particularly concerns an additive for such a fluid that will improve its seal swelling property concurrent with seal compatibility without at the same time imparting any detrimental effects thereto. The invention is also directed to additive concentrate packages that are intended for formulation into mineral oil base stocks to provide power transmission fluids of improved seal compatibility characteristics thereby enhancing fluid retention. These fluids have particular utility in a hydraulic fluid power steering mechanism.
A very high percentage of vehicles such as automobiles, tractors and earthmovers are now equipped with some type of semi-automatic or fully automatic transmission and/or power steering systems. These transmissions and systems must be proved with a supply of fluid that serves the multiple functions of a power transmitting medium, a hydraulic control fluid, a heat transfer medium, and a satisfactory lubricant. This fluid to be useful must be capable of operating over a wide temperature range, possess a high degree of oxidation resistance, be free of corrosive action, have foam control, have satisfactory low temperature fluidity, retain a useful viscosity at high temperatures, have transmission seal compatibility and lubricity without "stick-slip" of the transmission parts.
Power transmission fluids are probably one of the most complex automotive products offered by the lubricant supplier since the base oil contains one or more of the additive types: oxidation inhibitors; dispersants; metal deactivators; viscosity index improver; anti-wear agents; rust inhibitors; corrosion inhibitors; foam inhibitors; seal swellers; and friction modifiers (see Lubrication, Vol. 54, Nov. 1, 1968, pages 1-16). The seal swell agents must not only swell the seals sufficiently to contain the fluid within the transmission housing but must be compatible (as must the formulated transmission fluid) with the sealing materials which are generally nitrile, polyacrylate and isoprene elastomers. This compatibility is evidenced by retention of tensile strength and elongation without cracking after extended exposure to said fluids. The compatibility is even more critical in power steering applications where internal pressures of from 500 to 1000 psi are imposed on the seals (2500 to 5000% greater than forces found in automatic transmissions).
Since the current approach to improving transmission fluids is to adjust and modify the various additives so as to extend their useful lifetime, this approach is of little value unless the fluid is retained within the transmission system. Many of the automatic transmissions in use in vehicles at the present time lose significant quantities of transmission fluid, principally because of deterioration of the rubber-type seals or gaskets. As a result of this there have been numerous attempts to develop oil-soluble additives that can be formulated with or added to transmission, power steering and other hydraulic fluids to facilitate retention by suitably swelling and softening the rubber of elastomers in those seals and gaskets. The seal swell-soften additive should not lower the viscosity of the transmission fluid nor degrade its compatability with the sealing material. However, the essential need is to improve the transmission fluid, particularly that fluid originally placed in transmission, so that it is more completely and longer retained.
U.S. Pat. No. 3,389,088 teaches one class, i.e., aliphatic alcohols of 8 to 13 carbon atoms, e.g., tridecyl alcohol, of additives which perform to suitably swell the various gaskets and seals of automatic transmission systems. This type of additive has been improved on by its use in combination with an oil-soluble, saturated hydrocarbyl ester of 10 to 60 carbon atoms and 2 to 3 ester linkages, e.g., dihexyl phthalate (see U.S. Pat. No. 3,974,081).
Although not directed to "seal swelling" or "compatibility" requirements of hydraulic fluid compositions, several patents teach the combination of a phosphite and substituted phenol in fluid compositions having superior stability to oxidation:
U.S. Pat. No. 3,556,999 shows a major amount of lubricating oil containing a minor amount of each of C.sub.1 -C.sub.8 alkyl substituted phosphites, C.sub.1 to C.sub.4 alkyl substituted phenols or aromatic secondary amine and a dispersant copolymer containing N-vinyl-2-pyrrolidone; U.K. Pat. No. 1,282,652 teaches of a pump fluid containing from 0.2 to 2.0% of its weight of an organic antioxidant of the class consisting of phenols (preferably sterically hindered bisphenols), tris nonylphenyl phosphite and mixtures thereof; U.S. Pat. No. 3,115,465 teaches of a mixture of organic phosphite ester and methylenedbisphenol as an oxidation inhibitor for lubricating oils and for elastomers, including nitrile rubbers; and, Japan No. 75-4016 teaches the combination of a 2,2'thiobisphenol and tris(nonylphenyl) phosphite as an antioxidant for hot asphalt in storage.